Aluminum sacrificial anode

ABSTRACT

An extruded aluminum base sacrificial anode consisting essentially of about 0.02 to about 2 weight per cent bismuth, about 0.005 to about 0.05 weight per cent gallium and about 0.005 to about 0.5 weight per cent indium and a method to produce such anode are disclosed.

United States Patent [1 1 Reding et al.

ALUMINUM SACRIFICIAL ANODE Inventors: John T. Reding; Robert L. Riley,

Jr., both of Lake Jackson. Tex.

Assignee'. The Dow Chemical Company,

Midland, Mich.

Filed: July 15, 1974 Appl. No.: 488,334

US. Cl. 204/197; 72/364; 72/365; 72/377; 75/138; 204/148; 204/293 Int.Cl. C231 13/00 Field of Search 75/138; 204/148, 197, 293; 148/32;72/364, 365, 377

[451 Apr. 15, 1975 [56] References Cited UNITED STATES PATENTS 3,379,6364/1968 Reding et a1 75/138 Primary Examiner-R. Dean Attorney. Agent, orFirmRobert W. Selby 23 Claims, No Drawings ALUMINUM SACRIFICIAL ANODEBACKGROUND OF THE INVENTION This invention pertains to sacrificialanodes and more in particular to a method of forming an improvedaluminum alloy sacrificial anode.

Theoretically, aluminum should be expected to perform satisfactorily asa sacrificial anode because the element aluminum fulfills the twoprimary requirements for sacrificial anodes, that is, a high theoreticaloxidation potential (1.90 volts v. saturated KCl calomel reference) anda high theoretical electrical output per unit mass of metal consumed(2.98 amp.-hrs. per gram). In actual practice, however, unalloyedaluminum has not proven to be satisfactory for use as a sacrificialanode since it does not exhibit these favorable theoretical propertieswhen used as a sacrificial galvanic anode. The presence of the normallypassive oxide surface film on the aluminum apparently presents a barrierto the oxidation of the aluminum metal, thereby reducing the effectiveoxidation potential to about 0.7 volt (as measured in a closed circuitat either 250-r 1,000 milliamperes per square foot (ma/ft in a syntheticseawater electrolyte). Unless otherwise specified, all voltages hereinare with respect to a saturated potassium chloride (KCl)-calomel halfcell as a reference.

The effective oxidation potential of the aluminum metal in fresh wateris about 0.4 volt (as measured in a closed circuit at milliamperes persquare foot in water having a resistivity of about 5000 ohm.cm). Theeffective oxidation potential of aluminum in a saturated calcium sulfate(CaSO electrolyte is about 0.4 volt (as measured in a closed circuit ata current density of 50 milliamperes per square foot). At such lowoperating voltages, no effective cathodic protection is given to, forexample, ferrous based structures; therefore, the anode exhibits nouseful electrical output.

It is known in the art to add certain elements such as bismuth, gallium,indium, lead, magnesium, tin and zirconium to aluminum in an attempt toprovide an aluminum anode of commercial utility. Examples of suchaluminum alloys are illustrated in, for example, U.S. Pat. Nos.3,240,688; 3,337,332; 3,337,333; 3,368,952 and 3,379,636.

A method to produce an aluminum alloy suitable for use as an anode invarious environments including water heating systems and undergroundenvironments is desired.

SUMMARY OF THE INVENTION The novel method of the present inventioncomprises first providing an aluminum alloy consisting essentially ofabout 0.02 to about 2 weight per cent bismuth, about 0.005 to about 0.05weight per cent gallium, about 0.005 to about 0.5 weight per cent indiumand the balance of the alloy being essentially aluminum. The aluminumalloy is then hot worked sufficiently to provide a worked alloy suitablefor use as a galvanic anode with an oxidation potential of about 1.0 toabout l.3 volts at a current density of 10 ma/ft in a fresh waterelectrolyte with a resistivity of 5,000 ohm.cin or about 1.3 to 1.6volts at a current density of about 50 ma/ft in a saturated CaSOelectrolyte. The sacrificial anode of the present invention is useful inlow resistivity aqueous liquids and is especially useful for thegalvanic protection of ferrous members in, for example, water heaters orother aqueous environments having a resistivity of at least about 200ohms centimeter.

The method of forming the aluminum alloy galvanic anode comprises firsthot working a preferred alloy consisting essentially of about 0.03 toabout 0.3 weight per cent bismuth. about 0.005 to about 0.04 weight percent gallium, about 0.02 to about 0.3 weight per cent indium with thebalance of the alloy being essentially aluminum. This alloy contains thenormal impurities present in aluminum. The aluminum alloy is workedsufficiently to provide the desired oxidation potential. The hot workingcan be carried out by the known processes of drawing, forging, rollingand the like; however, it is preferred that the work be imparted intothe alloy by means of extrusion. Preferably the hot working is carriedout in a manner to provide a reduction in crossectional area from thestarting aluminum billet to the final worked anode of at least about 9to l and more preferably at least about 25 to l. The temperature of thesolid metal during hot working is at least about 200C. and preferablyfrom about 400 to about 600C.

The described alloy is preferably prepared by melting aluminum with apurity of at least about 99.5 weight per cent aluminum and then adding asufficient amount of the elements bismuth, gallium and indium to themolten aluminum to provide an alloy within the above defined compositionranges when the added elements are substantially uniformly dispersedwithin the aluminum. These elements can be readily dispersed in thealuminum by mixing equipment and methods commonly accepted in the art.Naturally the addition of aluminum, bismuth, gallium, and/or indiumalloys in amounts sufficient to form an aluminu m alloyp vvithin theherein described composition ranges is contemplated and included herein.To minimize the impurity level in the aluminum alloy anode or to producean aluminum base alloy consisting of bismuth, gallium and indium withinthe afore-specified ranges, it is desirable that the metal melted havean aluminum purity of at least about 99.7 weight per cent and preferablyat least about 99.85 weight per cent.

After the bismuth, gallium and indium are admixed with the moltenaluminum to provide the desired alloy, the molten metal is poured orcast into a suitable form or mold of a predetermined shape. The moltenalloy is solidified and removed from the mold. The as-cast shape, suchas an ingot, is heated to or maintained at a temperature sufficient forhot working of the metal. Preferably the temperature is sufficient toafford extrusion into a shape adapted for use as a galvanic anode in,for example, water heaters. The described worked alloy can be employedas a sacrificial anode using methods known to those skilled in the art.For example, attaching the anode to a more electropositive metalstructure, such as a steel member contained in a water heater, to affordan electrical contact between the anode and the steel causespreferential corrosion of the anode in corrosive environments.

As can be seen in the following examples and tables addition of bismuth,gallium and indium in the stipulatedjamounts to aluminum produces a hotworked sacrificial anode with a high useful voltage and a highcurrent-capacity (amp-hr. per pound output) in corrosive environments.

EXAMPLES 1-8 TABLE 1 Aluminum with a purity of 99.9 weight per cent wasmelted and heated to a temperature of 750C. Suffi- A .1 (M7) Pmemiulcient amounts of bismuth, gallium, and indium were 5 Ex "a?" M (mm) firdissolved in the molten aluminum to provide alloys with the compositionsshown in Table] after mixing if g 6 these elements 1nto the moltenalummum. The de- H (H17 (101 m 9 scribed alloys were cast into 2 V2 inchdiameter by 6 l2 (W2 700 13 0.07 0.02 0.12 1.5 885 inch long mgots. Thesol1d1f1ed mgots were removed (m7 (m4 (m2 L5 625 from the molds andheated to a temperature of 480C. 15 0.10 0.01 0.02 1.55 860 prior tobeing extruded into /2 inch diameter rod. The H extruded rod was cutinto about 7 inch long sections. 121 0.23 0.04 0.01 1.55 767 Theindividual sections were tested in an electrolyte {is 2%? comprising amixture of tap water and deionized water. 15 21 0:51 0:01 0:07 1:37 662The water had an electro-resistivity of 5.000 ohmcentimeters and atemperature of 70C. Each of the EXAMPLES 22-26 sections was immersed inthe aqueous electrolyte to a Aluminum with a purity of 99.9 weight percent was depth of l /2 inches and electrically attached to the meltedand heated to a temperature of 750"C. Sufflstainless steel container,which acted as the cathode. cient amounts of bismuth, gallium. andindium were The anode current density was approximately 10 ma/ft"dissolved in the molten aluminum to provide the alloy during testing.The voltage potentials shown in Table I compositions shown in Table 111after mixing these elewere measured with reference to a standardsaturated ments into the molten aluminum. The described alloys potassiumchloride-calomel half cell. were cast into 2 /z-inch in diameter by 6inch long ingot 25 and /8 inch in diameter by 6 inch long specimens. Thesolidified 2 A: by 6 inch ingots were removed from the TABLE 1 molds.heated to a temperature of 480C. and extruded into /2 inch diameter rod.The extruded rod was cut Measured Current into about 7 inch longspecimens. The individual ex- Analysis (wtfk) Potential p y truded andas-cast specimens were tested in saturated Ex B1 Ga in Al (volts)(t1mp.hrs/lh) caso1 aqueous electrolyte.

1 0.06 0.01 0.02 B111. 1.02 535 The extruded rods were immersed in theelectrolyte 2 440 to a depth of 3 inches and the as-cast specimens were3 0.07 0.02 0.08 1.15 480 4 x 7 4 2 I03 4 0 1mmersed to a depth Ofll'lCheS. extruded and 3-S- 5 022 cast test samples were electricallyconnected through 3 1. 21 8 a 18,200 ohm resistor to the positive sideof a rectifier. 8 0.31 0.01 0.12 1.15 405 Stainless steel rods wereconnected to the negative side of the rectifier and immersed in theelectrolyte to act as cathodes. The anode current density was approxi-40 mately 50 malft The voltage potential as shown in Table 111 wasmeasured with reference to a standard EXAMPLES 941 saturated KCl calomelhalf cell. It is readily apparent Specimens obtained substantially asdescribed in Exthat extrusion of the indicated alloys significantlyimamples l through 8 were tested in a saturated CaSO proved the anodecharacteristics of the alloys. aqueous electrolyte. Each specimen wasimmersed in EXAMPLES 27-58 the aqueous electrolyte to a depth of 3inches and elec- Aluminum base alloys with a composition as showntrically connected through an 18,200 ohm resistor to in Table IV wereprepared substantially as described in the positive side of a rectifier.Stainless steel rods were Examples 22-26. The anode current density wasabout connected to the negative side of the rectifier and im- 36 ma/ftThe data contained in Table IV represents mersed in the electrolyte toact as cathodes. The anode the anode characteristics after about 30 daysin the cor-. current density during testing was approximately 50 rosiveenvironment. It is apparent that the anode voltma/ft The voltagepotentials shown in Table ll were age potential and current capacity ofthe extruded anmeasured with reference to a standard saturated KC] odesare more uniform than and improved over the ascalomel half cell. castmaterial.

TABLE III AsCast Extruded Current Analysis (MT/r) Potential PotentialCapacity Example Bi Ga In Al (volts) (volts) (amp.hrs/lb) 22 0.1 0.010.01 Bal. 0.4 1.4 800 23 0.1 0.04 0.01 0.5 1.5 790 24 0.15 0.01 0.10 0.51.4 870 25 0.2 0.01 0.01 0.4 1.5 775 26 0.2 0.04 0.01 0.7 1.5 750 TABLEIV As-cast Extruded Current Current Analysis (wtf r 1 Pot. Capacity Pot.Capacity Examples Bi Ga ln (volts) (amp. hr/lb) (volts) (amp. hr/lh) 270.040 0.031 0.01 0.62 880 1.48 787 28 0.075 0.016 0.01 1.35 861 29 0.120.010 0.011 0.45 1ll l 1.45 800 30 0.12 0.023 0.015 1.34 794 1.50 647 310.12 0.011 0.12 1.51 430 1.57 693 32 0.13 0.011 0.020 0.50 1202 1.55 80433 014 0.005 0.012 1.31 995 1.52 769 54 0.15 0.011 0.01 0.5 93 1 1 55691 15 0.1 0.009 0.032 0.51 l 1 5 1.50 831 6 0.17 0.010 0.01 0.4 0051.55 736 3' 0.17 0.010 0.0116 0.55 1191 1.45 871 3h 0.17 0.018 0.0100.40 95*) 1 56 778 39 0.18 0.012 0.032 1.36 923 1.55 765 40 0.20 0.0090.01 0.50 l 195 1.51 794 41 0.20 0.024 0.01 0.46 94h 1.47 720 42 0.210.012 0.056 1.44 70,": 1.58 695 43 0.21 0.043 0.01 0.43 11 5 1.54 652 440.22 0.014 0.01 0.43 lll's'l 1.56 637 45 0.2. 0.019 0.010 0.39 956 1.5702 46 0.23 0.040 0.01 0.50 1 101 1.54 767 47 0.25 0.019 0.012 1.33 8801.57 737 48 0.26 0.008 0.011 0.46 l 168 1.51 776 49 0.27 0.013 0.01 0.411219 1.50 654 50 0.29 0.022 0.053 1.34 742 1.59 696 51 0.31 0.012 0.011.2 860 1.54 602 52 0.32 0.011 0.01 0.7 927 1.53 747 53 0.46 0.032 0.010.48 1020 1.49 599 5-1 052 0012 0.01 0.4 961 1.56 522 55 0.52 0.040 0.011.0 735 1.55 502 56 0.55 0.039 0.01 0.95 694 1.53 572 57 0.58 0.0200.012 1.3 7511 1.55 507 X 0.59 0.015 0.01 1.2 822 1.55 510 What isclaimed is:

1. An extruded sacrificial aluminum anode consisting essentially ofabout 0.02 to about 2 weight per cent bismuth. about 0.005 to about 0.05weight per cent gallium. about 0.005 to about 0.5 weight per cent indiumand the balance being essentially aluminum.

2. The extruded anode of claim 1 with an oxidation potential of about1.0 to about 1.3 volts at a current density of about 10 ma/ft in anaqueous electrolyte with a resistivity of about 5,000 ohm.cm.

3. The extruded anode of claim 1 with an oxidation potential of about1.3 to about 1.6 volts at a current density of about 50 Ina/ft in asaturated calcium sulfate aqueous electrolyte.

4. The extruded anode of claim 1 consisting essentially of about 0.03 toabout 0.3 weight per cent bismuth, about 0.005 to about 0.04 weight percent gallium and about 0.02 to about 0.3 weight per cent indium.

5. The extruded anode of claim 4 with an oxidation potential of about1.0 to about 1.3 volts at a current density of about 10 ma/ft in anaqueous electrolyte with a resistivity of about 5000 ohm.cm.

6. The extruded anode of claim 4 with an oxidation potential of about1.3 to about 1.6 volts at a current density of about 50 ma/ft in asaturated calcium sulfate aqueous electrolyte.

7. A method to form a sacrificial anode comprising providing an alloyconsisting essentially of about 0.02 to about 2 weight per cent bismuth,about 0.005 to about 0.05 weight per cent gallium, about 0.005 to about0.5 weight per cent indium and the balance being essentially aluminum;and working the alloy sufficiently to provide a reduction ratio of thecrosssectional areas of the starting alloy to the extruded anode of atleast about 9:1.

8. The method of claim 7 wherein the reduction ratio is at least about25 to l.

9. The method of claim 7 including working the alloy sufficiently toprovide a sacrificial anode with an oxidation potential of about 1.0 toabout 1.3 volts at a current density of about 10 ma/ft in an aqueouselectrolyte with a resistivity of about 5,000 ohm.cm.

10. The method of claim 7 including working the alloy sufficiently toprovide a sacrificial anode with an oxidation potential of about 1.3 toabout 1.6 volts at a current density of about 50 ma/ft in a saturatedcalcium sulfate aqueous electrolyte.

11. The method of claim 9 wherein the working is carried out byextrusion.

12. The method of claim 10 wherein the working is carried out byextrusion.

13. The method of claim 7 wherein the working is carried out byextrusion.

14. The method of claim 13 including heating the alloy to provide anextrusion temperature of from about 400C. to about 600C.

15. The method of claim 13 wherein the reduction ratio is at least 25 tol.

16. The method of claim 7 including heating the alloy to provide aworking temperature of at least about 200C.

17. The method of claim 7 wherein the alloy provided consistsessentially of about 0.03 to about 0.3 weight per cent bismuth, about0.005 to about 0.04 weight per cent gallium and about 0.02 to about 0.3weight per cent indium.

18. The method of claim 17 wherein the reduction ratio is at least about25 to 1.

19. The method of claim 17 wherein the working is carried out byextrusion.

20. The method of claim 19 including heating the alloy to provide anextrusion temperature of from about 400C. to about 600C.

21. The method of claim 20 wherein the reduction essentially aluminum. 7ratio is at least about 25 to 23. The alloy of claim 22 consistingessentially of aluminum alloy consisting essentially of about about 0.03to about 0.3 weight per cent bismuth, about to about 2 Weight per cent iabout 0005 0.005 to about 0.04 weight per cent gallium and about toabout 0.05 weight per cent gallium, about 0.005 to about 0.5 weight percent indium and the balance being to about welght per Cent mdlum' UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 5,878,081Dated p il 15, 1975 l t r( John T. Reding; Robert L. Riley, Jr'.

It 'is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 66, change "and" to -but-;

Column 4, line 50 after "the" second occurrence, insert saturated CaSOTable III,

Column 4, the word "Extruded" should be centered above columns 7 & 8.

Signed and Sealed this Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uflalenlsand Trademarks

1. AN EXTRUDED SACRIFICIAL ALUMINUM ANODE CONSISTING ESSENTIALLY OFABOUT 0.02 TO ABOUT 2 WEIGHT PER CENT BISMUTH, ABOUT 0.005 TO ABOUT 0.05WEIGHT PER CENT GALLIUM, ABOUT 0.005 TO ABOUT 0.5 WEIGHT PER CENT INDIUMAND THE BALANCE BEING ESSENTIALLY ALUMINUM.
 2. The extruded anode ofclaim 1 with an oxidation potential of about 1.0 to about 1.3 volts at acurrent density of about 10 ma/ft2 in an aqueous electrolyte with aresistivity of about 5, 000 ohm.cm.
 3. The extruded anode of claim 1with an oxidation potential of about 1.3 to about 1.6 volts at a currentdensity of about 50 ma/ft2 in a saturated calcium sulfate aqueouselectrolyte.
 4. The extruded anode of claim 1 consisting essentially ofabout 0.03 to about 0.3 weight per cent bismuth, about 0.005 to about0.04 weight per cent gallium and about 0.02 to about 0.3 weight per centindium.
 5. The extruded anode of claim 4 with an oxidation potential ofabout 1.0 to about 1.3 volts at a current density of about 10 ma/ft2 inan aqueous electrolyte with a resistivity of about 5000 ohm.cm.
 6. Theextruded anode of claim 4 with an oxidation potential of about 1.3 toabout 1.6 volts at a current density of about 50 ma/ft2 in a saturatedcalcium sulfate aqueous electrolyte.
 7. A method to form a sacrificialanode comprising providing an alloy consisting essentially of about 0.02to about 2 weight per cent bismuth, about 0.005 to about 0.05 weight percent gallium, about 0.005 to about 0.5 weight per cent indium and thebalance being essentially aluminum; and working the alloy sufficientlyto provide a reduction ratio of the cross-sectional areas of thestarting alloy to the extruded anode of at least about 9:1.
 8. Themethod of claim 7 wherein the reduction ratio is at least about 25 to 1.9. The method of claim 7 including working the alloy sufficiently toprovide a sacrificial anode with an oxidation potential of about 1.0 toabout 1.3 volts at a current density of about 10 ma/ft2 in an aqueouselectrolyte with a resistivity of about 5,000 ohm.cm.
 10. The method ofclaim 7 including working the alloy sufficiently to provide asacrificial anode with an oxidation potential of about 1.3 to about 1.6volts at a current density of about 50 ma/ft2 in a saturated calciumsulfate aqueous electrolyte.
 11. The method of claim 9 wherein theworking is carried out by extrusion.
 12. The method of claim 10 whereinthe working is carried out by extrusion.
 13. The method of claim 7wherein the working is carried out by extrusion.
 14. The method of claim13 including heating the alloy to provide an extrusion temperature offrom about 400*C. to about 600*C.
 15. The method of claim 13 wherein thereduction ratio is at least 25 to
 1. 16. The method of claim 7 includingheating the alloy to provide a working temperature of at least about200*C.
 17. The method of claim 7 wherein the alloy provided consistsessentially of about 0.03 to about 0.3 weight per cent bismuth, about0.005 to about 0.04 weight per cent gallium and about 0.02 to about 0.3weight per cent indium.
 18. The method of claim 17 wherein the reductionratio is at least about 25 to
 1. 19. The method of claim 17 wherein theworking is carried out by extrusion.
 20. The method of claim 19including heating the alloy to provide an extrusion temperature of fromabout 400*C. to about 600*C.
 21. The method of claim 20 wherein thereduction ratio is at least about 25 to
 1. 22. An aluminum alloyconsisting essentially of about 0.02 to about 2 weight per cent bismuth,about 0.005 to about 0.05 weight per cent gallium, about 0.005 to about0.5 weight per cent indium and the balance being essentially aluminum.23. The alloy of claim 22 consisting essentially of abouT 0.03 to about0.3 weight per cent bismuth, about 0.005 to about 0.04 weight per centgallium and about 0.02 to about 0.3 weight per cent indium.